The present invention relates generally to preparation of carboxylic acid esters and specifically to a method for preparing alkoxy alkyl esters.
The present invention is directed to preparation of carboxylic acid esters and specifically to a method for preparing alkoxy alkyl esters of alkylene glycol monoalkyl ethers in a continuous mode. The reaction liberates water which in addition to unreacted reactants causes operational and purification problems.
In continuous processes to produce esters by reaction of an alcohol and a carboxylic acid, the water of reaction is removed to increase conversion. Typically, the reaction is carried out using a reactor containing a mixture of alcohol, carboxylic acid, ester, water, and an acid catalyst. The reactor is heated to obtain an equilibrium mixture and the products distilled in a fractionating column. As product is distilled alcohol and carboxylic acid are fed to the reactor. With simple esters such as ethyl acetate and butyl acetate the water is removed as azeotropes with the ester and unconverted alcohol. The distillate separates into two liquid phases. The upper phase, referred to as the xe2x80x98oil phasexe2x80x99, contains mainly ester with a little alcohol and some water. The lower phase, referred to as the xe2x80x98water phasexe2x80x99, contains mostly water with some ester and alcohol. The water phase is transferred to a distillation tower and the water discharged from the bottom of the tower as waste; the distillate is recycled. The oil phase is distilled in a purification tower to produce a base discharge product of pure ester and a distillate which is recycled to the reactor. This process has been optimized over the years to allow production of these simple esters at high rates.
When it is attempted to esterify alkylene glycol monoalkyl ethers such as 1-methoxy-2-propanol using this process it has been found to work poorly, if at all. The distillate from the reactor column does not readily separate into two phases, making it very difficult to remove the water of reaction by the above process. The reason that phase separation does not occur is that the alcohol and ester are much more soluble in water compared to simple ester. It has been found that it is possible to operate the reactor/distillation tower in such a manner so as to separate two closely boiling azeotropes (one richer in the alkylene glycol monoalkyl ether and the other richer in the corresponding ester). Although this can be accomplished by operating the distillation tower at a high reflux to distillate ratio, operation of the distillation tower in this manner greatly decreases its capacity. This produces a distillate, which does separate into an oil phase and a water phase, but the degree of separation is poor. Furthermore, the reactor/distillation tower must be operated at such a low rate to make the overall production economically unfeasible.
Because of the solubility problem described above, the alkylene glycol monoalkyl ether esters are usually manufactured by a process described in European Patent Application 0119833 B1. A compound such as toluene is added to the reactor and the water is removed by distillation as an azeotrope with toluene. This drives the reaction to completion. The azeotrope separates into two phases; the water is removed as the water phase and the oil phase is recycled to the reactor. In this process only water is distilled as the azeotrope, leaving ester, unreacted alcohol or carboxylic acid, and catalyst in the reactor. This process requires removal of the catalyst from the product by neutralization or some other means prior to purification. Another drawback is that these processes are normally run in batches rather than in a continuous mode and result in low raw material efficiencies and loss of catalyst. Moreover, ester made this way tends to have problems with acidity and stability.
It is an object of the present invention to overcome the foregoing difficulties. We have discovered that an alkylene glycol monoalkyl ether ester, such as 1-methoxy-2-propyl acetate can be produced in a continuous process in high yield, at high rate, with excellent product quality, and without catalyst loss. This is accomplished using water as the azeotropic agent and distilling the product as an azeotrope of water, carboxylic acid ester, and some unreacted glycol ether alcohol from the reactor into an overhead decanter/extractor. This product results in a single phase. A small amount of an inert solvent is fed to the decanter/extractor causing the distillate to separate into an oil phase and a water phase. The oil phase contains primarily the solvent, ester, and a small amount of water and reacted alcohol. The water phase contains primarily water and unreacted alcohol and some ester. Unreacted carboxylic acid and catalyst remain in the reactor. This process is not constrained by the difficulty in separating closely boiling azeotropes or higher boiling ester products and results in substantially higher production rates. Moreover, the carboxylic acid is not distilled overhead and does not contaminate the product.